Have Recent Improvements in Air Quality Eliminated the Summertime Warming Hole in the Southeast United States?

During the twentieth century, as the rest of the world warmed, the Southeast US experienced an anomalous "warming hole" characterized by negative temperature trends (between 1900 and 2008). The existing literature attributes much of this ``warming hole" to increased anthropogenic aerosol concentrations in the atmosphere during the latter-half of the century and the ability of these aerosols to directly and indirectly cool the climate. Here we use satellite observations from the Multi-angle Imaging Spectroradiometer (MISR) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instruments as well as a one-dimensional radiative transfer model to diagnose how major reductions in regional particulate concentrations since 2001 (but especially since 2008), both at the surface and aloft, affected the warming hole during the Northern Hemisphere summer. While twenty-first century aerosol concentrations have generally declined across the Continental United States, the reductions were most steep in the Southeast US and we attribute the elimination and reversal of the warming hole during the twenty-first century to noticeably improved air quality in the region. Our results also inform the analyses of data obtained during the summer of 2013 SEAC4RS field campaign.